Dental erosion involves demineralization and damage to the tooth structure due to acid attack from nonbacterial sources. Erosion is found initially in the enamel and, if unchecked, may proceed to the underlying dentin. Dental erosion may be caused or exacerbated by acidic foods and drinks, exposure to chlorinated swimming pool water, and regurgitation of gastric acids. The tooth enamel is a negatively charged surface, which naturally tends to attract positively charged ions such as hydrogen and calcium ions, while resisting negatively charged ions such as fluoride ions.
Dentinal hypersensitivity is acute, localized tooth pain in response to physical stimulation of the dentine surface as by thermal (hot or cold) osmotic, tactile combination of thermal, osmotic and tactile stimulation of the exposed dentin. Exposure of the dentine, which is generally due to recession of the gums, or loss of enamel, frequently leads to hypersensitivity. Dentinal tubules open to the surface have a high correlation with dentine hypersensitivity. Dentinal tubules lead from the pulp to the cementum. When the surface cementum of the tooth root is eroded, the dentinal tubules become exposed to the external environment. The exposed dentinal tubules provide a pathway for transmission of fluid flow to the pulpal nerves, the transmission induced by changes in temperature, pressure and ionic gradients.
The prior art discloses the use of various stable, soluble zinc-amino acid-halide complexes for the treatment of dentinal hypersensitivity and improved oral health. When placed in oral care formulations, these complexes provide an effective concentration of zinc ions to the enamel, thereby protecting against erosion, reducing bacterial colonization and biofilm development, and providing enhanced shine to the teeth. Moreover, upon use, these formulations provide a precipitate that can plug the dentinal tubules, thereby reducing the sensitivity of the teeth. These formulations have the added benefit that they do not exhibit the poor taste and mouthfeel, poor fluoride delivery, and poor foaming and cleaning associated with conventional zinc-based oral care products using soluble zinc salts.
Of particular interest are compositions comprising the zinc-amino acid-halide complex zinc-lysine-chloride complex, designated ZLC, which may be formed from a mixture of zinc oxide and lysine hydrochloride. ZLC has the chemical structure [Zn(C6H14N2O2)2Cl]+Cl−, and may exist in solution of the cationic cation ([Zn(C6H14N2O2)2Cl]+) and the chloride anion, or may be a solid salt, e.g., a crystal, optionally in mono- or dihydrate form.
While oral care compositions comprising the ZLC complex are known, it has been challenging to formulate compositions comprising this complex which have good long-term stability. It has been particularly difficult to formulate low-water dentifrice compositions comprising ZLC complex, because these compositions suffer from unacceptably high increases in viscosity during storage. This progressive thickening or gelation makes the composition unsuitable for consumer use. There is thus a need for improved formulations comprising ZLC complex.